1. Technical Field
The present invention relates to a measurement circuit and a test apparatus.
2. Related Art
In a test apparatus for testing a device under test, a comparator is used to convert an output signal from the device under test into a digital signal by comparing the level of the output signal to a reference level, as shown in Patent Document 1, for example.    Patent Document 1: Japanese Patent Application Publication No. 2001-99894
The logic value output by the comparator changes when the magnitude relationship between the level of the input signal and the reference level changes. The response time, from when this magnitude relationship between the input signal level and the reference level changes until when the logic value output by the comparator changes, depends on the overdrive amount, which is the amount by which the input signal level exceeds the reference level.
More specifically, a greater overdrive amount results in the comparator having a faster response time. Therefore, the response time of the comparator changes according to the signal level input thereto, and so the signal input to the comparator cannot always be accurately converted into a digital signal.
One idea for solving this problem involves decreasing the absolute value of the response time variation by improving the gain of the comparator to obtain an even faster response time for the comparator. However, the response time variation cannot be reduced to zero. Therefore, problems can occur when inputting a high-frequency signal into the comparator, for example. Furthermore, there is a limit to how fast the response time can become as a result of improving the comparator capability.